UNITED NATIONS (AP) — U.S. Ambassador Nikki Haley urged the U.N. peacekeeping force in Lebanon on Monday to step up efforts to prevent the spread of illegal arms in the south, which she said "are almost entirely in the hands of Hezbollah terrorists."

Haley made clear in a statement that the United States is seeking "significant improvements" to the U.N. force, known as UNIFIL, when the U.N. Security Council renews a mandate that is due to expire Aug. 31.

Her statement was in response to a letter to the Security Council from Secretary-General Antonio Guterres saying he intends to look at ways in which UNIFIL could "enhance its efforts."

But the U.N. chief stressed that the Lebanese armed forces have primary responsibility for ensuring that "there are no unauthorized armed personnel, assets or weapons" in the southern area between the Litani River and the U.N.-drawn Blue Line separating Lebanon and Israel where UNIFIL operates.

"On its part, UNIFIL, in coordination with the Lebanese armed forces, remains determined to act with all means available within its mandate and capabilities on concrete information provided regarding the illegal presence of armed personnel, weapons or infrastructure inside its area of operations," Guterres said.

Israel has long complained that Hezbollah militants operate freely in the south.

Haley said the United States, a strong ally of Israel, "will continue to raise the threat posed by Hezbollah as we seek significant improvements to UNIFIL when the Security Council renews its mandate this month."

The secretary-general said that despite the "long period of relative calm and stability in southern Lebanon and along the Blue Line," neither Israel nor Lebanon have fulfilled their obligations under the Security Council resolution that ended the Israeli-Hezbollah war in 2006.

"Israel must withdraw its forces from Lebanese territory and stop violations of Lebanese airspace," Guterres said.

"The government of Lebanon must exercise effective authority over all Lebanese territory, prevent hostile actions from its territory, ensure the safety and security of the civilian population, in addition to United Nations personnel, and also ensure the disarmament of all armed groups," he said.

Ensuring that there are no weapons or "authority" in the south except the Lebanese government and army is vital "in moving from the fragile cessation of hostilities to a permanent cease-fire," Guterres said.

He urged all parties to use the current relative calm to take "positive steps" toward a permanent cease-fire and long-term solution to the conflict.